Evaluation of moment-to-force characteristics of preactivated symmetrical t-loop made of two different alloys: An in-vitro study

This in-vitro study evaluated the Moment-to-Force characteristics of symmetrical T-loops with different preactivations, constructed from superelastic NiTi wires and compared it with those of TMA wires. The study consisted of ninety samples of symmetrically placed T-loops made of two different alloys. Samples were divided into three groups, Group A: TMA, Group B: NiTi (furnace), Group C: NiTi (SMAS). Each group comprised of thirty samples. Ten samples for each degree of preactivation for all three groups were tested. Samples were constructed from commercially available 0.018 x 0.025 inch austenitic NiTi wire (3M Unitek) and 0.017 x 0.025 inch TMA wire (Ormco). The T-loop used had the following dimensions - height 7mm, width 10 mm and radius 2mm. Preactivation bends of 0°, 15° and 30° were given at the junction of the vertical arm and the occlusal horizontal arm of the closing loops on both sides. Two methods were used for the fabrication of NiTi T - Loops: 1. After forming the loop, it was incorporated in Orthocal and heated at 510°c for 9 minutes in a muffle furnace. 2. Using SMAS, a Shape memory alloy shaper (Jaypee General Agencies) which is a pulsed-heat induction device. The measurements were taken for each sample at an interval of 1mm from the neutral position during the activation and deactivation range to maximum of 7 mm for TMA and 8 mm for NiTi closing loops. A custom made loop testing apparatus (IIT Chennai) was used to measure the forces and moments generated. The following conclusions were drawn: 1. The non preactivated T- loops failed to produce an optimum M:F for theoretical tooth movement via translation. 2. With increasing degrees of preactivation, the M:F also increased over the deactivation range for all the T-loops. 3. The M : F generated by NITi loops were greater than TMA T-loops over the deactivation range. 4. NiTi T-loop produced lower force levels which is considered as biologically acceptable, over its activation and deactivation range compared with the equivalent TMA T-loop specimens. 5. NiTi T-loops allowed greater range of activation and the M : F generated was high enough to produce translation and later root movement over a greater range of deactivation. This can be advantageous in the clinical scenario as in a premolar extraction case where complete space closure can be achieved with minimum adjustments. 6. Even though there is significant difference in the M:F generated by the two NiTi groups, both the groups provide sufficient M:F for bodily movement of teeth if appropriate preactivations are given. SMAS can be further modified to simplify the fabrication of NiTi T-loops and makes chair side adjustment of the NiTi closing loops possible. Further studies are required to confirm these findings